It is known to connect such framework to a source of direct current to apply a voltage sufficient to maintain the metal in a corrosion resistant state, to avoid or treat corrosion. Such a system is called cathodic protection.
Generally this technique utilises a distributed anode system such as metal mesh, or conductive coatings such as paints and speciality gunited and sprayed material. There are however some areas which cannot be addressed in this way and "discrete" or "point" anodes are suggested. There are already two basic types of point anodes on the market. the most common being a platinised titanium rod. The other type is generally a shaped titanium mesh or titanium metal tube or plate, catalysed with an appropriate or mixed metal oxide based material.
The platinised titanium rod cannot be used alone however, because at the current ratings required (which are controlled by the density of the steel bars in the vicinity), the current density at the anode surface would be very high, generating acid at a rate far faster than it could diffuse away (and ultimately be neutralised by the alkali generated on the cathodic reinforcing bars). Hence the concrete would be destroyed by acid attack in the neighbourhood of the anode. The US National Association of Corrosion Engineers have a guideline that the current density should be no higher than 100 A./m.sup.2 of concrete area to avoid this phenomenon. The rod is usually put in a significantly larger hole (typically 12 mm diameter) than would otherwise be required and the void is filled with a carbon based conductive paste to increase the surface area and hence reduce the current density at the concrete surface. For cost reasons however they still tend to be run well above the NACE guidelines.
At current densities above 0.3 mA.cm length (about 800 mA/m.sup.2) the carbon backfill is consumed by anodic oxidation forming CO.sub.2. As a result the contact between the carbon and the concrete is lost and the voltage to drive the protecting current rises, often outstripping the rating of the rectifier. In addition to this though, other mechanisms are possibly taking place. The high current flows generate gases, typically oxygen with traces of chlorine or carbon dioxide from the oxidation of the carbonaceous paste at the anode surface and this may have difficulty escaping, causing blowholes further reducing the contact at the concrete surface. (It is known that at very high current densities (around 15 Am.sup.2) the anodes can "gas block" within hours of being powered up.)
Generally, because of the need to attend to a specialised conductive carbonaceous grout, the rod is restricted to anodes near the surface. Often there is steel work buried deep within the structure which would be assisted with a local anode close by.